A theoretical study of factors influencing the microbial population dynamics of the activated-sludge process—II. A computer simulation study to compare two methods of plant operation

The effects of operating a plant with a constant sludge-wastage rate and with a constant concentration of mixed-liquor solids have been investigated by computer simulations. From a theoretical point of view, if any sewage parameter is not completely stable then a slightly better quality effluent is likely to be achieved when the plant is operated with a constant specific wastage rate. In practice, however, the slight improvement is not likely to be measurable. The effects of a sudden stable increase or a “slug dose” of sewage substrate and bacterial content on the plants operated in the above two ways have been simulated together with diurnal variations in sewage flow, and in the concentrations of bacteria, debris, and substrate in the sewage.